Tin (Sn)

Tin holds a record that is not about strength, melting point or price: it has ten stable isotopes, more than any other element in the periodic table. The runner-up, xenon, has nine, and most elements manage one, two or three.

This is not a coincidence of arithmetic. Fifty is a magic number in nuclear physics — a proton count that exactly fills a shell, in the same way that a filled electron shell makes a noble gas chemically content. A closed proton shell binds the nucleus more tightly and makes it tolerant of a wide range of neutron counts, so tin remains stable from 112 neutrons-plus-protons all the way to 124. Its neighbours indium and antimony, one proton either side, manage two natural isotopes each — and in indium's case only the rarer of the two is genuinely stable. Tin's abundance of them is a direct readout of nuclear structure.

Stannum, and a symbol from the wrong metal

The English name descends from a Germanic root shared with German Zinn and Dutch tin, and it is old enough that its origin before that is unrecoverable. The symbol comes from Latin stannum, which is the interesting half of the story, because stannum did not originally mean tin. In earlier Latin it referred to an alloy of silver and lead, or to the lead-silver mixture skimmed from smelting. Only later did the word shift to the metal we now call tin, displacing the older Latin plumbum candidum, "white lead".

That drift left a permanent trace. Tin's chemistry is described with the words stannous and stannic, minerals are cassiterite and stannite, and the ancient Cornish courts governing the tin trade were the Stannaries — all from a word that first meant something else.

The metal that always had to be imported

Bronze is roughly nine parts copper to one part tin, and it is enormously better than copper alone: harder, castable, capable of holding an edge. The difficulty is that copper deposits are widespread and tin deposits are not. Cassiterite, the oxide ore, forms in association with granites, and across the ancient Near East — the region where bronze metallurgy was most intensive — there is essentially none.

Every Bronze Age palace economy therefore depended on tin arriving from somewhere far away, and tracking where has become one of archaeology's better detective stories. Tin and lead isotope work on ingots recovered from Late Bronze Age wrecks off the coast of Israel points to a source in the Variscan tin belt of western Europe, with the Cornish deposits the most likely candidate — tin from the edge of the Atlantic reaching the eastern Mediterranean in the thirteenth century BC. Ingots from the Uluburun wreck off southern Turkey and from Mochlos on Crete appear to have a different origin, with Central Asian sources implicated. The picture that emerges is not one road but several, feeding a single indispensable commodity into a network that collapsed when the network did.

Cornwall kept producing for three thousand years afterwards. South Crofty, the last working tin mine in Europe, closed in 1998.

What happens to tin in the cold

Ordinary white tin is a metal. Below about 13.2 °C it is thermodynamically unstable with respect to grey tin, a brittle grey semiconductor with the diamond structure and a density around 20% lower. The transformation makes the metal swell, crumble and fall apart as grey powder, and it is called tin pest.

In practice it is very slow near the transition temperature and needs deep cold and a long time to get started, and it can be suppressed entirely by small amounts of alloying. Two famous anecdotes attribute historical disasters to it — the buttons on Napoleon's army in Russia, and the fuel cans that leaked on Scott's Antarctic expedition — and neither stands up to scrutiny. Where tin pest is documented beyond argument is in cold European churches, where the tin pipes of old organs have grown grey warts, and in low-temperature testing of modern lead-free solders.

Bending a bar of tin produces an audible crackling, the tin cry, as crystal twins shear past one another. It is the loudest example of an effect that indium and a few other soft metals share more faintly.

Whiskers, and the cost of removing lead

Pure tin electroplate spontaneously grows single-crystal filaments out of its own surface. They are a few micrometres across, can reach several millimetres long, grow over months or years without any applied voltage, and conduct. On a circuit board with fine pin spacing they cause short circuits that appear at random and often vaporise, leaving no evidence.

Adding a few per cent lead to the tin suppresses whisker growth almost completely, which is why the phenomenon was a curiosity for most of the electronics era. Restrictions on lead in electronics from the mid-2000s removed that protection, and tin whiskers became a live reliability problem across the industry. They have been implicated in satellite failures, including the loss of a commercial communications satellite in 1998, in nuclear plant relay faults, and in medical device recalls, and mitigation now relies on alloying, conformal coatings and nickel underlayers rather than on any complete solution.

Solder, cans and a bath of molten metal

Roughly half of world tin production goes into solder, which makes electronics assembly by far the element's largest market. The rest divides among a handful of uses that all exploit tin's low melting point, its resistance to corrosion, or both:

• Tinplate. A "tin can" is steel with a tin coating a fraction of a micrometre thick, applied electrolytically. The tin is what stops the steel reacting with the contents, and the technology is old enough that the canning of food predates any understanding of why food spoils.
• Float glass. The Pilkington process floats molten glass on a bath of molten tin, which is the only metal that is liquid across the required temperature range, denser than glass, and chemically unreactive towards it. The bath makes both surfaces perfectly flat with no grinding, and it is how essentially all window glass in the world is made.
• Chemicals. Organotin compounds stabilise PVC against degradation during processing, and tin compounds catalyse the polyurethane reaction.
• Alloys. Bronze, pewter and bearing metals continue, at modest volumes.

The paint that changed the sex of sea snails

Tributyltin, painted onto ship hulls from the 1960s, was the most effective antifouling agent ever deployed — it kept barnacles and weed off so thoroughly that fleets saved measurable percentages of fuel.

It was also an endocrine disruptor at concentrations of parts per trillion. In marine gastropods it induced imposex: female dog whelks and other snails grew male organs and became sterile, and populations collapsed near harbours and shipping lanes across the world. Oyster fisheries in France reported deformed shells and failed spat. The International Maritime Organization banned the application of organotin antifoulants from 2003 and their presence on hulls from 2008, making it one of very few chemicals prohibited globally by treaty on ecological grounds alone.

Bangka, Wa State, and the conflict-mineral list

Modern tin comes chiefly from China, Indonesia, Myanmar, Peru and Bolivia. Indonesian production is concentrated on the islands of Bangka and Belitung, where a large part of the output is artisanal, much of it dredged from shallow sea floor by divers working from small pontoons, with a fatality record that has drawn sustained criticism. Myanmar's supply comes overwhelmingly from the Man Maw area in Wa State, outside central government control; when the Wa authorities suspended mining in 2023, world tin prices moved within days.

Tin is the first T in the 3TG group — tin, tantalum, tungsten and gold — subject to conflict-minerals due diligence rules, because cassiterite from eastern Democratic Republic of the Congo has financed armed groups. It is a curious continuity: the metal whose supply lines were strategic in 1300 BC is on a list of strategic supply lines today, for recognisably similar reasons.